Compensatory Release of Ultrashort-Chain Perfluoroalkyl Substances at the Water-Soil Interface Following Post-Drought Rehydration in Paddy Soil
Ultrashort-chain perfluoroalkyl substances (PFAS) exhibit high hydrophilicity, mobility, and root concentration factors, facilitating their transport and accumulation in soil-crop systems and posing phytotoxicity risks. Post-drought rehydration (PDR) is a critical water management strategy to mitigate drought effects in paddy fields. This study investigated the regulation and mechanisms of PDR on ultrashort-chain PFAS transport in paddy soils through sterilized and non-sterilized experiments, employing three-dimensional fluorescence spectroscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, and amplicon sequencing. Results showed that PDR increased the bioavailable fraction of ultrashort-chain PFAS in soil solution while delaying their release into overlying water. Sterilization experiments confirmed that PDR-induced compensatory migration was primarily driven by microbial activity. Geochemical analyses revealed that PDR reduced hydrophilic functional groups (e.g., hydroxyl) on soil particle surfaces and increased cation bridging sites. Microbiological sequencing indicated that PDR activated secondary metabolic pathways, enhancing microbial extracellular polymeric substances (EPS) production, which provided binding sites for ultrashort-chain PFAS. Consequently, EPS competed with soil particles for cation bridging, altering PFAS interfacial partitioning and increasing bioavailable and cation-complexed fractions in soil solution, thereby exacerbating rhizosphere exposure risk to rice. This study elucidates the coupled geochemical and microbiological mechanisms governing ultrashort-chain PFAS mobility under PDR, informing risk assessment and management in paddy agroecosystems.